In recent years, there has been a trend towards the combination of different medical imaging modalities for improved diagnosis. For example, the integration of biochemical or metabolic imaging provided by positron emission tomography (PET) and anatomical imaging provided by computed tomography (CT) has proven itself as a standard clinical tool. However, the use of CT in combination with PET has its limitations, such as notable irradiation (X-ray) dose and reduced soft tissue contrast compared to magnetic resonance imaging (MRI). For this reason, MRI is now considered to be an improved alternative to CT. Moreover, the combination of PET and MRI has the potential to offer truly simultaneous acquisition of PET and MRI data. This allows for simultaneous dynamic studies of different parameters, such as diffusion and perfusion, to gain new diagnostic information. In addition, it also enables an enhanced PET image reconstruction, for example, by correcting the acquired PET data for the effects of patient movements, because global and local motion of a body can be tracked efficiently by means of MRI. However, in order to benefit from the combination of two different medical imaging modalities, such as PET and MRI, one must be able to temporally relate the acquired data to each other.